bfloyd

I understand that the general correct fit for saddle fore and aft is determined by the front of the knee in line with the pedal spindle when the cranks are parallel with the ground. Now, I notice that seat tube angles can vary between 73 and 74 degree (the most common from what I've seen) and at times 75 degree. How does this affect the knee in line with axle theory? Is this where set back seatposts come into play? How much of a set back would the seat post need to go from a 74 degree seat tube to make it more like a 73 degree? Thanks.

lvleph

I wonder if anyone will actually answer your question. I just don't think about my fit that much. I set things so that I feel comfortable when riding. Eventually, I will have everything dialed, and then it will be time for a new bike.

Longfemur

The seat tube angle is totally meaningless except as a way to get the saddle where you want it. It doesn't affect anything whatsoever, except to the extent that if you need your saddle more forward than usual (as for a tri bike, for example), then you will need the seat tube to be less laid back or else you don't have enough adjustment range to allow you to move the saddle that far forward. Same with rearward. For example, if you have unusually long thighs, than you might not be able to get the saddle far back enough unless the seat tube is more laid back. In both cases, you can achieve the same thing with an appropriately offset seat post if you really need to (but of course, the rest of the bike is built around the angles it has, and so, a seriously offset seatpost could potentially affect the rider's weight distribution on the bike adversely).

Lots of people dismiss the knee over pedal rule, but the thing is, it's not nor has it ever been a rule. It's just a rule of thumb to get you started more or less in the right ballpark.

kuan

There are only three points of contact. Your butt, your feet, and your hands. Imagine yourself in the perfect position, then move the stem and seat accordingly.

The purpose of a steep seat tube is to facilitate comfort in a more aerodynamic position so your knees don't hit your chest. It also prevents your legs from moving beyong their range of motion, ie., preventing you from squatting too low. A steeper tube might also mean you might have to use a longer stem so you're not too crunched.

Campag4life

Floyd, your assumptions are pretty close. If you have a short inseam, a more upright seat post of say 75 deg. allows you to achieve KOPS without much if any setback to your post. If you are long legged like me, then you want a frame with a 72 deg. seat tube...not easy to fnd btw...to achieve KOPS without a fair amount of setback. As mentioned KOPS is somewhat of a benchmark but no where near an absolute. Many recreational cyclists prefer being just behind KOPS because it takes the weight off their hands because of their more ordinary pedaling forces. If you have long femurs, you need a small sta to get the knee closer over and not in front of the pedal spindle.
HTH.

bfloyd

Thanks all.